The present invention relates to active materials for fuel cell anodes, and more specifically, hydrogen storage alloy active materials for the anode of Ovonic instant startup/regenerative fuel cells. The active material includes a hydrogen storage alloy material with an additive, which, upon utilization of the active material in an anode of an alkaline electrolyte fuel cell, gives the anode added benefits, not attainable by using hydrogen storage alloy material alone. These added benefits include 1) precharge of the hydrogen storage material with hydrogen; 2) higher porosity/increased surface area/reduced electrode polarization at high currents; 3) simplified, faster activation of the hydrogen storage alloy; and optionally 4) enhanced corrosion protection for the hydrogen storage alloy. These benefits are achieved by adding a water reactive chemical hydride to the hydrogen storage alloy used as the active material of the negative electrode of the alkaline fuel cell.
As the world""s human population expands, greater amounts of energy are necessary to provide the economic growth all nations desire. The traditional sources of energy are the fossil fuels which, when consumed, create significant amounts of carbon dioxide as well as other more immediately toxic materials including carbon monoxide, sulfur oxides, and nitrogen oxides. Increasing atmospheric concentrations of carbon dioxide are warming the earth; creating the ugly specter of global climatic changes. Energy-producing devices which do not contribute to such difficulties are needed now.
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Highly efficient fuel cells employing hydrogen, particularly with their simple combustion product of water, would seem an ideal alternative to current typical power generations means. Researchers have been actively studying such devices to utilize the fuel cell""s potential high energy-generation efficiency.
The base unit of the fuel cell is a cell having a cathode, an anode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Presently most of the fuel cell R and D is focused on P.E.M. (Proton Exchange Membrane) fuel cells. Regrettably, the P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, the electrolyte for the system is acidic. Thus, noble metal catalysts are the only useful active materials for the electrodes of the system. Unfortunately, not only are the noble metals costly and of limited availability, but they are also susceptible to poisoning by many gases, specifically carbon monoxide (CO). Also, because of the acidic nature of the P.E.M fuel cell electrolyte, the remainder of the materials of construction of the fuel cell need to be compatible with such an environment, which again adds to the cost thereof. The proton exchange membrane itself is quite expensive, and because of it""s low proton conductivity at temperatures below 80xc2x0 C., inherently limits the power performance and operational temperature range of the P.E.M. fuel cell as the PEM is nearly non-functional at low temperatures. Also, the membrane is sensitive to high temperatures, and begins to soften at 120xc2x0 C. The membrane""s conductivity depends on water and dries out at higher temperatures, thus causing cell failure. Therefore, there are many disadvantages to the P.E.M. fuel cell which make it somewhat undesirable for commercial/consumer use.
The conventional alkaline fuel cell has some advantages over P.E.M. fuels cells in that they have higher operating efficiencies, they use less costly materials of construction, and they have no need for expensive membranes. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, platinum active materials are used in conventional alkaline fuel cell electrodes. Unfortunately, conventional alkaline fuel cells still suffer from their own disadvantages.
For example, conventional alkaline fuel cells still use expensive noble metal catalysts in both electrodes, which, as in the P.E.M. fuel cell, are susceptible to gaseous contaminant poisoning. The conventional alkaline fuel cell is also susceptible to the formation of carbonates from CO2 produced by oxidation of the anode carbon substrates or introduced via impurities in the fuel and air used at the electrodes. This carbonate formation clogs the electrolyte/electrode surface and reduces/eliminates the activity thereof. The invention described herein eliminates this problem from the anode.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the anode for hydrogen oxidation and the cathode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous anode and cathode and brought into surface contact with the electrolytic solution. The particular materials utilized for the cathode and anode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the anode occurs between the hydrogen fuel and hydroxyl ions (OHxe2x88x92) present in the electrolyte, which react to form water and release electrons:
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92E0=xe2x88x920.828 v. 
At the cathode, the oxygen, water, and electrons react in the presence of the cathode catalyst to reduce the oxygen and form hydroxyl ions (OHxe2x88x92):
O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92E0=xe2x88x920.401 v. 
The total reaction, therefore, is:
2H2+O2xe2x86x922H2O E0=xe2x88x921.229 v 
The flow of electrons is utilized to provide electrical energy for a load externally connected to the anode and cathode.
It should be noted that the anode catalyst of the alkaline fuel cell is required to do more than catalyze the reaction of H+ ions with OHxe2x88x92 ions to produce water. Initially the anode must catalyze and accelerate the formation of H+ ions and exe2x88x92 from H2. This occurs via formation of atomic hydrogen from molecular hydrogen. The overall reaction may be simplified and presented (where M is the catalyst) as:
M+H2xe2x86x922M . . . Hxe2x86x92M+2H++2exe2x88x92. 
Where M . . . H denotes atomic hydrogen adsorbed on the catalyst. Thus the anode catalyst must not only efficiently catalyze the electrochemical reaction for formation of water at the electrolyte interface, but must also efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional anode material, the dissociated hydrogen is transitional and the hydrogen atoms can easily recombine to form hydrogen if they are not used very efficiently in the oxidation reaction. With the hydrogen storage anode materials of the inventive instant startup fuel cells, hydrogen is stored in hydride form as soon as they are created, and then are used as needed to provide power.
In addition to being catalytically efficient on both interfaces, the catalytic material must be resistant to corrosion by the alkaline electrolyte. Without such corrosion resistance, the electrode would quickly succumb to the harsh environment and the cell would quickly lose efficiency and die.
One prior art fuel cell anode catalyst is platinum. Platinum, despite its good catalytic properties, is not very suitable for wide scale commercial use as a catalyst for fuel cell anodes, because of its very high cost, and the limited world supply. Also, noble metal catalysts like platinum, also cannot withstand contamination by impurities normally contained in the hydrogen fuel stream. These impurities can include carbon monoxide which may be present in hydrogen fuel or contaminants contained in the electrolyte such as the impurities normally contained in untreated water including calcium, magnesium, iron, and copper.
The above contaminants can cause what is commonly referred to as a xe2x80x9cpoisoningxe2x80x9d effect. Poisoning occurs where the catalytically active sites of the material become inactivated by poisonous species invariably contained in the fuel cell. Once the catalytically active sites are inactivated, they are no longer available for acting as catalysts for efficient hydrogen oxidation reaction at the anode. The catalytic efficiency of the anode therefore is reduced since the overall number of available catalytically active sites is significantly lowered by poisoning. In addition, the decrease in catalytic activity results in increased over-voltage at the anode and hence the cell is much less efficient adding significantly to the operating costs. Overvoltage is the difference between the actual working electrode potential under some conditions and its equilibrium value, the physical meaning of overvoltage is the voltage required to overcome the resistance to the passage of current at the surface of the anode (charge transfer resistance). The overvoltage represents an undesirable energy loss which adds to the operating costs of the fuel cell.
In related work, U.S. Pat. No. 4,623,597 (xe2x80x9cthe ""597 patentxe2x80x9d) and others in it""s lineage, the disclosure of which is hereby incorporated by reference, one of the present inventors, Stanford R. Ovshinsky, described disordered multi-component hydrogen storage materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made (i.e., atomically engineered) to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, intermediate range order, and/or polycrystalline (lacking long range compositional order) wherein the polycrystalline material includes topological, compositional, translational, and positional modification and disorder. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the ""597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed the batteries to be utilized most advantageously as secondary batteries, but also as primary batteries.
Tailoring of the local structural and chemical order of the materials of the ""597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the anodes of the ""597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. Disorder permits degrees of freedom, both of type and of number, within a material, which are unavailable in conventional materials. These degrees of freedom dramatically change a materials physical, structural, chemical and electronic environment. The disordered material of the ""597 patent have desired electronic configurations which result in a large number of active sites. The nature and number of storage sites were designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provided a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods there between resulting in long cycle and shelf life.
The improved battery of the ""597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the ""597 patent, be chemically modified with other elements to create a greatly increased density of electro-catalytically active sites and hydrogen storage sites.
The disordered materials of the ""597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The internal topology that was generated by these configurations also allowed for selective diffusion of atoms and ions. The invention that was described in the ""597 patent made these materials ideal for the specified use since one could independently control the type and number of catalytically active and storage sites. All of the aforementioned properties made not only an important quantitative difference, but qualitatively changed the materials so that unique new materials ensued.
Disorder can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
These same principles can be applied within a single structural phase. For example, compositional disorder is introduced into the material which can radically alter the material in a planned manner to achieve important improved and unique results, using the Ovshinsky principles of disorder on an atomic or microscopic scale.
Additionally, in copending U.S. application Ser. No. 09/524,116, (""116), the disclosure of which is hereby incorporated by reference, Ovshinsky has employed the principles of atomic engineering to tailor materials which uniquely and dramatically advance the fuel cell art. The invention of ""116 application has met a need for materials which allow fuel cells to startup instantaneously by providing an internal source of fuel, to operate in a wide range of ambient temperatures to which a fuel cell will be exposed to under ordinary consumer use and to allow the fuel cell to be run in reverse as an electrolyzer thereby utilizing/storing recaptured energy. The anodes of the ""116 fuel cells are formed from relatively inexpensive hydrogen storage materials which are highly catalytic to the dissociation of molecular hydrogen and the formation of water from hydrogen and hydroxyl ions as well as being corrosion resistant to the electrolyte, resistant to contaminant poisoning from the reactant stream and capable of working in a wide temperature range.
Thus, Ovonic Regenerative Fuel Cells use metal hydrides that are capable of reversibly absorbing/desorbing hydrogen from the gas phase as well as electrochemically. Both these reactions should have fast kinetics and be reversible in order to sustain the reaction continuously. The hydrogen gas entering from the gas side of the anode will continuously hydride the Ovonic alloy to form metal hydrides. At the same time continuous electrochemical discharge from the electrolyte side will deplete the hydride regenerating the Ovonic alloy. The hydriding and dehydriding are a dynamic processes that need to occur efficiently to keep the fuel cell working well.
One problem encountered in the actual running of the cell involves activation or forming of the electrodes. Conventional activation of any hydride former is accomplished by repeatedly absorbing and desorbing hydrogen under pressure. In using the Ovonic Metal Hydrides in fuel cells, this cannot be done by the conventional method because the cells are not designed to stand high pressures or temperatures. Hence an electrochemical pulsing is used as a method of activating the electrodes on the gas and electrolyte side simultaneously. This pulsing involves quick bursts of electrochemical charging and discharging for several cycles. The gas side is continuously supplied with hydrogen during this formation. While this procedure works, it involves a fair amount of time to accomplish the desired level of activation. It also involves assembling, disassembling and reassembling of the working and counter electrodes. This disassembly and reassembly inevitably exposes the activated electrodes to atmosphere leading to oxidation. Mechanical integrity of the electrode is also adversely affected by the assembly and disassembly operations.
There are a few options to solve this problem. They are: 1) prehydride the powders before making the electrodes; 2) prehydride the electrodes; and 3) externally charge the electrodes electrochemically and assemble the cell. These options are not practical because: 1) prehydrided powders cannot be handled safely and are also susceptible to oxidation; 2) prehydrided electrodes will lose their integrity in the prehydriding chambers and are also highly susceptible to oxidation; and 3) externally charged electrodes will have both sides exposed to electrolyte thus compromising the integrity of the hydrophobic sealing layer. It is also difficult to prevent them from oxidizing.
Thus what is needed is a practical method of precharging/activating the electrode with hydrogen and electrode materials for performing such precharging/activation.
The present invention is a hydrogen storage alloy active material for the negative electrode of Ovonic instant startup/regenerative fuel cells. The active material includes a hydrogen storage alloy material with an additive, which, upon utilization of the active material in an anode of an alkaline electrolyte fuel cell, gives the anode added benefits, not attainable by using hydrogen storage alloy material alone. These added benefits include 1) precharge of the hydrogen storage material with hydrogen; 2) higher porosity/increased surface area/reduced electrode polarization at high currents; 3) simplified, faster activation of the hydrogen storage alloy; and 4) optionally, enhanced corrosion protection for the hydrogen storage alloy. These benefits are achieved by adding a water reactive chemical hydride to the hydrogen storage alloy used as the active material of the negative electrode of the alkaline fuel cell.
An example of such a chemical hydride is sodium borohydride. Other chemical hydrides, such as alkali and alkaline earth metal hydrides may be used. Examples of such hydrides are LiAl hydrides (LiAlH6), KH, NaH, and CaNi5H6. All of these chemical hydride will serve the same basic purpose. The chemical hydrides are chosen such that they are unstable in the presence of water and react therewith to produce hydrogen. Preferably the chemical hydride is stable in air. The chemical hydride should be selected such that its reaction byproduct with water minimally has no deleterious effect on the fuel cell operation and may be selected such that its byproduct has a beneficial effect (such as corrosion inhibition) on the fuel cell. In any event, the byproduct should at least be inert to the fuel cell""s main chemical reactions for converting hydrogen and oxygen into water and energy.